1. Field of Application
This invention relates to valves for the human heart and more particularly to a prosthetic heart valve.
2. Description of the Prior Art
In the field of heart valves there have been many attempts to produce a valve that is similar or substantially similar to the natural heart valve. The reason is that if the replacement valve differs in any substantial way from the natural valve, its reaction in the heart muscle might be so detrimental as to cause failure of the heart itself. One approach is known as the Starr-Edwards valve. It utilizes vertical struts, a steel or plastic ball movably entrapped in cage and having a larger diameter than the opening in a circular ring. The pressure of blood flowing through the ring moves the ball away from the ring and permits blood flow through the valve. The reversal of pressure causes the ball to be seated again against the ring to block flow through the valve. There are many problems with the Starr-Edwards type valve. The flow is not axial since it must flow around the ball. Such a flow quite often creates a very serious problem since it tends to cause turbulence (eddy currents) which restricts the flow of blood and may cause blood clots. The presence of blood clots in turn leads to thromboembolisms and improper seating of the valve, and anticoagulants must be used continually. The projecting cage member can cause scarring of the heart wall; the clicking sound of the valve can be emotionally disturbing.
There has been some work done in attempting to produce a valve which more closely simulates the shape and axial flow characteristics of the natural heart valve, and, of course, eliminates the ball. Most of these have been directed to tricuspid or three lip valves.
Tricuspid valves may require the use of certain heart valves from swine. One of the problems encountered is that hundreds of pigs have to be slaughtered before one or two perfect valves can be found. Secondly, in a tricuspid valve, as will be explained hereinafter, if there is slight damage or shrinkage to one of the cusps, the valve does not close properly. Also, swine valves are not particularly large and because of mounting problems, only about 30% of the area of the mounting ring can be used for the opening in the valve. Thus, much of the internal diameter of the normal heart valve is lost with this type of valve.
Moreover, tricuspid valves have an inherent problem in that the three lips must close in exact alignment or the valve does not properly close. If one lip is slightly out of line or misshapen, the other two lips cannot make up for the difference and, therefore, leakage occurs. Likewise, after the valve has been used for some period of time, one of the cusps may become slightly inflexible and immobile, causing tissue to build up on that cusp and further reduce its mobility. The result is stenosis or total lack of movement of the cusp. This reduces the size of the opening and the amount of blood flowing through the valve. Also, the stenosis may cause turbulence in the flow of blood and improper closure which almost invariably leads to failure of the valve. Some more of the problems and description of many types of heart valves are discussed in detail in the Journal of Thoracic and Cardiovascular Surgery, Vol. 68, No. 3, September, 1974, pages 261 to 269 and by applicant in Vol. 76, No. 6, December, 1978, pages 771 to 787.
There has been some effort to produce a two cusp valve which attempts to simulate the shape and flow of the natural heart valve. However, the principal problems with such a valve (as shown in U.S. Pat. No. 3,739,402 to Cooley et al.) are the lack of an adequate volume of blood flow through the valve and the inability to obtain complete closure.
This patent specifies that the lips can only open to approximately 2 millimeters, or only open to a small fraction (20%) of the valve passage area. Thus, only 20% of the amount of blood entering the valve can exit and the result is a tremendous strain on the heart and the valve, which may possibly lead to failure of both. Reduction in flow also causes high back pressure within the heart and may seriously damage the inside of the heart.
The inflexibility of the lips of the device disclosed in the Cooley et al. patent causes them to close along a thin narrow line at their leading edge. This closing creates two serious problems. With a narrow line of closure, the slightest deformity in either lip prevents complete closing and causes leakage through the valve. Second, since the valve closes only along this narrow line, the valve is often unable to remain closed when subjected to the extreme pressures in the heart. This is especially important when the valve is used in the mitral position in the heart, since the pressure differential across the valve in that position is substantial, and any leakage could lead to cardiac failure.
A heart valve is needed which closely simulates the natural heart valve by having the flexibility to open to substantially the same size as the natural opening and the flexibility to completely close and to remain closed against the force of large pressure differentials.
In my copending U.S. application, Ser. No. 841,791, filed Oct. 13, 1977, I have disclosed an improved two cusp heart valve with a semiflexible support ring and two lips which closely simulates a natural heart valve. Still, it would be beneficial to more closely simulate a natural heart valve by providing a more flexible support.
It is, therefore, an object of this invention to provide a prosthetic heart valve which closely resembles the natural heart valve.
It is a further object of this invention to provide a heart valve having full axial laminar flow through a tubular member.
A still further object of this invention is to provide a heart valve having sufficient flexibility to open to the full diameter of the valve and to fully close and remain completely closed.
Another object of this invention is to provide a heart valve which moves in conjunction with the annulus of the opening in which it is placed.
Still another object of this invention is to provide a heart valve which is strong enough to resist the pressure of the heart without failure.
Yet still another object of this invention is to provide a heart valve having proper closure and minimal resistance to flow.
Another object of this invention is to provide a heart valve with a single movable lip.
To the above ends, I have provided a prosthetic valve for the heart comprising a tubular membrane having an inlet opening at one end adapted to attach the valve to the annulus of a heart and an outlet opening at a second end. A support structure is adapted to attach the membrane to the heart for preventing the second end from protruding through the inlet opening when the valve closes.